Varistors are commonly made by sintering particulate metal oxides, with or without minor amounts of other inorganic materials, as in disk or rod form. They have non-linear electrical resistance over a range of applied voltages, often only slightly conductive at customary power line voltage but increasingly conductive at higher voltages. Varistors are frequently interposed between an upstream power source and downstream electrical equipment powered from that source for protection of such load equipment against over-voltages. Varistors function by shunting to ground the brief but extremely high currents resulting from transient voltage spikes present on the power lines, such as may result from lightning or faulty switching. See my U.S. Pat. Nos. 4,866,560; 4,901,187; 4,907,119; and 4,931,895; which illustrate diverse arrangements of varistors providing TVSS.
Sintered varistor compositions are so hard that it is hardly practical to subdivide such a structure into thin wafer-like units. In conventional practice, particulate constituents are die-pressed together into individual self-supporting form and then are sintered.
An alternative is the so-called "thick film" varistor, which is formed in place from similar particulate material, whether dry or in paste form, as on a non-conductive substrate, which al so usually carries connecting terminals. In such varistors electrical conduction occurs mainly along--rather than perpendicular to--the thin varistor dimension. Examples of such varistors in U.S. patents are disclosed by Jefferson in U.S. Pat. No. 3,916,366 and also by Aoki et al. in U.S. Pat. No. 4,333,861; whereas examples of suitable varistor compositions appear in such U.S. Pat. Nos. as 4,045,374; 4,211,994; and 4,794,048.